We have developed a new method for investigating quantitative aspects of medical decision-making. Thresholds are the probabilities of disease at which a clinician will decide to use a diagnostic test or to initiate therapy. The threshold concept has previously been advanced as a means of calculating theoretically optimal probabilities to guide decision-making. These threshold probabilities can be obtained using decision analytic techniques. In contrast, we describe a method of deriving a physician's (or group of physicians) actual thresholds governing clinical decisions, a descriptive rather than normative approach. Our preliminary studies have shown differences in the use of diagnostic tests for patients with suspected coronary artery disease by specialty type of physicians. The threshold model should also be useful for examining the importance of different types of patients. We now have a unique opportunity to apply the threshold model to decisions involving patients with suspected pulmonary embolism. Under the direction of Dr. Eugene Saenger, the Society for Nuclear Medicine has completed a study of the clinical usefulness of lung scans. The data base includes over two thousand patients and contains the information that is needed for applying the threshold model. Because of the willingness of Dr. Saenger to collaborate, we can apply a useful model to an important problem for which the clinical data has already been collected. We will use decision making thresholds to investigate the role of patient characteristics in physicians' management of patients with suspected pulmonary embolism. Specifically, we will test the hypotheses that physicians have higher thresholds for testing and treating: 1. elderly patients compared to younger patients, 2. patients with known cancer versus those who do not have cancer, 3. patients with mild versus severe symptoms of pulmonary embolism, 4. patients without insurance than for those with insurance. This research will provide a greater understanding of physicians' decision making and their use of diagnostic tests for patients with suspected pulmonary embolism.